In many of the more popular mobile telephony systems, such as AMPS and TACS systems, a mobile station itself determines the cell onto which it locks, by scanning a defined number of frequencies, which are control channels for access purposes, and then choosing that frequency which has the highest received signal strength in the mobile station. The available frequencies in these mobile telesystems are often permanently distributed between the cells and the distribution is often made so that the frequencies can be reused. A normal procedure in this regard is to distribute the frequencies over an area which includes 21 cells, with this 21-cell pattern being later repeated. This distribution pattern enables cells of mutually the same frequencies to be spaced so far apart as to exclude essentially the interference of one cell by another cell.
The AMPS and TACS systems use a control channel for establishing connections and traffic channels for the conversation phase of a connection. The systems normally use 21 frequencies as control channels, based on the popular 21-cell repetition pattern, with one control channel per cell.
The functions of a control channel can be divided into two logical function areas: paging and access. In addition to paging messages to mobile stations with which calls shall be established, the paging channel also includes a general information function, a so-called broadcast information function. Broadcast information may also have an effect on access. In many cases, the paging and access functions lie on one and the same frequency, even in analogue systems such as the aforementioned AMPS and TACS systems. A control channel which has these functions is hereinafter referred to as a combined PAC control channel, ie a paging and access control channel.
In some instances, 21 control channel frequencies can be thought to be too few, for instance in troublesome interference cases where 21 frequencies are insufficient to avoid control channel interferences between the cells. In these instances, it is possible to increase the number of combined PAC control channels up to a total of 32, so as to enable other frequency repetition patterns to be used. Alternatively, it is possible to continue to use the 21-cell pattern while having a number of frequencies in reserve for the event of needing to solve a frequency planning problem in certain troublesome areas.
Naturally, both the paging and the access channel has a limited capacity. The requisite capacity of such channels will depend to a large extent on subscriber behaviour, cell size, etc., but can also be determined by the operator to a large degree, for instance by limiting the paging area over which a paging message is transmitted, or by how often the mobile stations are forced to register themselves.
A high control channel capacity requirement is normally reflected in a high traffic channel requirement, this traffic channel requirement being resolved by adding more cells. When each cell is equipped with a combined